From English to Greek translation both are accepted. The main difference between these two is the time/chronological reference:
- Γράφε: is imperative to write when we know that the action will take some time or we cannot count how much time it will take or how often it will occur until it is considered completed.
(teacher in a classroom) Σταμάτα να μιλάς και γράφε = Stop speaking and write
- Γράψε: It should take just few seconds/minutes or it is an action that won't have continuity
- (between friends) Γράψε το τηλέφωνο του = Write down his phone
Mainly γράψε is an action that will occur only once, while γραφε indicates an action that will occur more than once or it is simply uncountable at that moment.
Σταμάτα να μιλάς is in fact in singular and would be addressed to one student.
For many people Σταματήστε would be used: Σταματήστε να μιλάτε.
Now, this seems inconsistent because σταμάτα is the present imperative but σταματήστε the past (See conjugation table for χτυπάω here http://users.sch.gr/ipap/Ellinikos%20Politismos/Yliko/Theoria%20Nea/klisi-rimatos-NE.htm).
The correct form to use is the past imperative (σταμάτησε/σταματήστε) because stopping happens in a moment, not continuously, and σταμάτησε is indeed used. However, σταμάτα is also used, in violation of the grammar rule, probably because it's shorter and oftentimes such grammatical differences do not really register and wrong usage is established. In the same manner, σταματάτε is used out of place, but usually with a negation: Δε σταματάτε να μιλάτε επιτέλους;! Αρκετά! (Why) Dοn't you stop talking already?! Enough!
I can't think of a verb being stressed on the last syllable in the imperative, unless it's only one syllable of course, e.g. δες/see! βγες/get out! (both singular, plural being δείτε, βγείτε respectively).
Stress, as in accent marks, is placed on a syllable as a matter of grammar, like spelling, not for emphasis. Accent placement does not depend on intonation. It is grammatically fixed and there is only one way to pronounce a word correctly. So when an accent moves from one syllable to another from the nominative to the genitive case for example, this is because of a grammatical rule.
I think the problem here lies with the TTS recordings. Because of the presence of the exclamation mark, the voice goes a bit higher in the end, which is not very accurate. I suggest listening to the audio for the word on its own page (click on it) which does not include the exclamation mark. You can also check native speakers' recordings on forvo.com or use a better, in my opinion, free TTS tool available on http://www.acapela-group.com/ where you can type longer texts.